Bone is a dynamic tissue that is continually remodeled throughout life depending on factors such as nutrition and the load the bone must carry. Normal bone formation depends on the delicate balance between new bone addition and old bone resorption. Bone formation is based on the deposition of bone matrix by osteoblasts and bone resorption and more specifically mineralized tissue, chiefly calcium carbonate and calcium phosphate resorption in vertebrates is achieved by osteoclasts. Typically, in a normal adult, about 5-10% of bone is replaced by these processes annually.
These osteoclasts are multinucleated cells of up to 400 μm related to macrophage and other cells that develop from monocyte cells, which are actively motile cells that migrate along the surface of bone. Like macrophage, osteoclasts are derived from haematopoietic progenitor cells. The bone resorption is initiated when an osteoclast attaches to the surface of mineralized bone, forms a tight “sealing zone” and secretes necessary acids and proteases that initiate the resorption of mineralized tissue from the bone. After a period of several hours to days, the osteoclast detaches from the bone, leaving a pit on the bone surface. Under normal conditions, the pit is a target for osteoblasts, which deposit a material that ultimately becomes new bone.
Bone loss can result when the bone resorptive process is dominant over the bone formative process. Diseases associated with bone loss are usually accompanied by increased osteoclast activation. Such diseases include any bone loss resulting notably from an estrogen deficiency after the menopause but not only and comprise osteoporosis, osteopenia due to bone metastases, periarticular erosions in rheumatoid arthritis, primary hyperparathyroidism, hypercalcemia of malignancy, Paget's disease of bone, periodontal disease, immobilization induced osteopenia, and glucocorticoid treatment.
As an example, there are currently 20 million people with detectable fractures of the vertebrae due to osteoporosis in the United States. In addition, there are 250,000 hip fractures per year attributed to osteoporosis. This clinical situation is associated with a 12% mortality rate within the first two years, while 30% of the patients require nursing home care after the fracture.
Since diseases of bone loss are associated with increased activity of osteoclast, it is important to understand the mechanisms by which osteoclasts are activated in these disease states, and to devise rational and therapeutic means to inhibit or reduce this activation.
Thus, the aim of the present invention is to elaborate new screening methods which can be useful for treating and/or preventing bone loss diseases, and to use such compounds to prepare a drug for treating and/or preventing bone loss diseases.